A matrix-assisted laser desorption ionization time-of-flight (MALDI- TOF) mass spectrometer is required for both ongoing and proposed studies to be carried out by approximately 20 NIH-funded investigators at the Medical College of Wisconsin and Blood Research Institute of Wisconsin. The proposed instrument will become part of the Protein and Nucleic Acid Shared Facility of the Medical College and will be operated in the Facility by Dr. Jasper Chu, an investigator with 5 years mass spectrometry experience characterizing proteins and peptides. The MALDI-TOF mass spectrometer will be used mainly for accurate and sensitive mass analysis and structure determination of proteins and peptides and of derivatives of proteins and peptides including covalent enzyme-inhibitor adducts, phosphorylated and glycosylated derivatives, and derivatives bearing modifications of N-terminal amino acids. The instrument will also be used for analysis of large protein complexes and for the identification of proteins from 2D gels. We have determined that the MALDI-TOF mass spectrometer system best meets these combined needs. Among its advantages are: (i) high mass range for proteins (>100kD), (ii) high sensitivity (fmol range), (iii) ability to sequence peptides using post-source decay spectra and "ladder sequencing," and (iv) ability to analyze complex sample mixtures.